1. Field of the Invention
The field of the invention is programmable controllers, and more particularly, apparatus for monitoring various conditions within such controllers during their operation, and for indicating any malfunctions which may occur.
2. Description of the Prior Art
One common type of programmable controller includes a controller processor unit that is connected through communication cables to one or more I/O interface racks. The I/O interface racks are connected to corresponding sets of operating devices, which either directly connect to a machine or directly control an industrial process. The controller processor unit includes a memory and operates in response to a control program stored in the memory to examine the status of sensing devices and to energize and deenergize output devices according to the sensed conditions and the logic contained in the control program. Such sensing devices might include, for example, limit switches and photoelectric cells, while such output devices might include solenoids, relays and motor starters. Besides controlling these operating devices, the programmable controller also monitors conditions within its own components to detect malfunctions that may occur.
In programmable controllers, such as that disclosed in Struger et al, U.S. Pat. No. 4,118,792, issued Oct. 3, 1978, and entitled "Malfunction Detection System for a Microprocessor-Based Programmable Controller," there are two possible responses to malfunctions detected in the controller. One response is an interruption in communication between the controller processor and the I/O interface racks. The controller processor is held, and all operating devices connected to the interface racks are decontrolled. The other response is the activation of a fault status indicator on the component of the programmable controller in which the fault originated. In the programmable controller referred to above, light-emitting diodes (LEDs) are used for this purpose.
In the prior controller referred to above, major faults such as a communication fault between the controller processor and any of the I/O interface racks cause the interruption of communication and the illumination of a LED on the I/O interface rack where the fault originated. For major faults in the controller processor itself, a watchdog timer "times out," the operation of the programmable controller is suspended, and a LED on the controller processor unit is illuminated. For minor fault conditions, such as a weak memory back-up battery, the controller processor is not interrupted, but a LED on the controller processor is illuminated. In programmable controllers having read/write random-access memories (RAMs), a memory back-up battery is necessary for supplying power to the memory when a controller is not being operated, so that the contents of the memory will not be altered or lost.
With the introduction of programmable controllers having I/O interface racks distributed to locations that are remote from the controller processor unit (see, for example, Markley et al, U.S. Pat. No. 3,997,879, issued Dec. 14, 1976), a minor fault indication at the controller processor is not likely to be observed by machine operators at the I/O interface racks. In many industrial applications, programmable controllers are run nearly continuously and unattended, so that the controller processor unit is observed only at irregular intervals. The LEDs on the controller processor unit will only be observed during startup and shutdown, which may only occur when the controller processor is serviced. It would therefore be advantageous to have fault status indicators on the I/O interface racks, which indicate those controller processor faults that do not result in a complete suspension of operation, e.g., a LED indicating a weak memory back-up battery.